This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In studies performed during our previous proposals, a new methodology was developed for analyzing and interpreting iron Ledge transition intensity distributions in terms of the total and differential orbital covalency (DOC). It was found that the integrated iron L-edge intensity could be used to obtain the total covalency of the complex. Furthermore, a projection methodology was developed which allows the covalency of the individual symmetry-related sets of orbitals and the DOC to be experimentally determined from the L-edge multiplet intensity distribution. The previous study serves as a foundation for the series of studies proposed here. Building on previous studies of back-bonding in low-spin heme complexes, a series of high-spin heme complexes will be understood in terms of the L-edge multiplet structure. This work, combined with the L-edge of low-spin heme, will be used to help spectroscopally resolve long-standing issues involving the binding of O2 to hemoglobin. These heme studies will then be applied to determining the electronic structure in a series of complexes that serve as models of the enzyme cytochrome c oxidase. The determination of covalency and DOC of these models will provide insight into the electronic structure of this enzyme and differences related to the O-O bond cleavage. Next, because the enzymatic turnover in many non-heme iron enzymes is thought to involve a highvalent Fe(IV)=O intermediate, our understanding of the iron L-edge will be use to look at a series of S=1 and S=2 Fe(IV)=O model complexes known to perform H-atom abstraction. We will then extend these studies to series of high-valent binuclear model complexes with the goal of obtaining mechanistic insight into the active site methane monooxygenase. Finally we will use iron Ledge XAS to understand the nature of the Fe-NO bond as NO in relation to reactions with O2.